1. Field of the Invention
The present invention relates to an activation chamber and a kit used in a treatment device for lowering electron affinity; a treatment device containing the kit and used to lower electron affinity; a photocathode electron-beam source; an electron gun containing a photocathode electron-beam source; a free-electron laser accelerator containing a photocathode electron-beam source; a transmission electron microscope containing a photocathode electron-beam source; a scanning electron microscope containing a photocathode electron-beam source; an electron-beam holography microscope containing a photocathode electron-beam source; an electron-beam lithography device containing a photocathode electron-beam source; an electron-beam diffraction device containing a photocathode electron-beam source; and an electron-beam scanning device containing a photocathode electron-beam source; and relates in particular to an activation chamber used in a treatment device for lowering electron affinity, treatment to lower the electron affinity of a photocathode material, and treatment to again lower the electron affinity of a photocathode, can be carried out repeatedly within a single vacuum chamber without the need to separately furnish a vacuum chamber for treatment to lower the electron affinity of a photocathode material and treatment to again lower the electron affinity of a photocathode material, and an electron-beam source chamber in which a photocathode is arranged and emits electrons, as in the prior art, and with which emission of electrons can be carried out; a treatment device containing the kit and used to lower electron affinity; a photocathode electron-beam source; an electron gun containing a photocathode electron-beam source; a free-electron laser accelerator containing a photocathode electron-beam source; a transmission electron microscope containing a photocathode electron-beam source; a scanning electron microscope containing a photocathode electron-beam source; an electron-beam holography microscope containing a photocathode electron-beam source; an electron-beam lithography device containing a photocathode electron-beam source; an electron-beam diffraction device containing a photocathode electron-beam source; and an electron-beam scanning device containing a photocathode electron-beam source.
2. Description of the Related Art
To date, electron-beam sources that utilize GaAs semiconductor photocathodes (GaAs photocathode electron-beam sources) have contributed to the field of accelerator science, as sources for spin-polarized electron beams having high polarization, in elementary particle/hadron physics experiments (precision measurement of Weinberg angle), or as high-brightness electron-beam sources capable of high-repetition short pulses of a high-current beam to generate a 1 kW infrared free-electron laser, and the like.
Further, GaAs photocathode electron-beam sources are strong candidates as high brightness electron-beam sources capable of producing a low-emittance (the area occupied by the beam within phase space) high-current beam for use in next-generation radiation source accelerators, and according to the “International Linear Collider Project,” which is a future project for a linear, next-generation accelerator to help unravel the mysteries behind the creation of the universe, are considered as the sole practical high-performance spin-polarized electron sources.
Meanwhile, together with detailed structural analysis and elemental analysis on an atomic scale, intra-structural measurements of electrical and magnetic characteristics are considered crucial for the purposes of miniaturization of semiconductor devices and development of advanced functional materials. Next-generation observation and measurement techniques that exceed existing capabilities will be needed to meet this demand, and development of high-capability electron-beam sources, which is an elemental technique, will be crucial in this regard. GaAs photocathode electron-beam sources are viewed as powerful candidates as electron-beam sources for use in next-generation electron microscopes, due to their high-repetition rate and short pulse width, high brightness, and high spin polarization capabilities.
A GaAs photocathode electron-beam source utilizes a negative electron affinity (hereinafter sometimes denoted as “NEA”) surface (a state in which the vacuum level is below the bottom of the conduction band). By utilizing an NEA surface, electrons that have been photoexcited from the valence band to a potential level at the bottom of the conduction band can be output as-is into a vacuum as an electron beam. FIG. 1 shows the concept of generating an electron beam from a GaAs photocathode electron-beam source, and can be explained in terms of the phenomenology of the three-step model: (1) excitation, (2) scattering, and (3) escape, described next. (See non-patent document 1)
(1) Excitation light impinges on a photocathode, exciting valence band electrons into the conduction band (excitation).
(2) The electrons excited into the conduction band scatter to the surface (scattering).
(3) Electrons reaching the surface tunnel through the surface barrier and escape into the vacuum (escape).
A GaAs semiconductor has electron affinity (the energy differential between the vacuum level and the conduction band bottom) of about 4 eV, and the following process is necessary to form an NEA surface state.
(1) First, a p-type doped GaAs semiconductor is heated in a vacuum, and surface impurities such as oxides and carbides are removed to clean the surface. In so doing, band bending is produced in a surface region, and the vacuum level is lowered to about half the bandgap (φB) of the semiconductor.
(2) Next, as shown in FIG. 2, in order to obtain a very small photocurrent on the crystal surface, first, cerium is vapor deposited thereon, and then cesium deposition and oxygen addition are repeated alternately until maximum photocurrent is obtained, for each saturation of photocurrent. By lowering the remaining vacuum level (φD) by this method, an NEA surface state can be formed (see Non-Patent Document 1).
The NEA surface state refers to a state in which, through the aforedescribed process, the energy level of the vacuum level of the photocathode is brought to a state lower than the energy level of the conduction band bottom. However, electrons can be emitted into a vacuum even when the energy level of the vacuum level of the photocathode is higher than the energy level of the conduction band bottom. Moreover, even after treatment to bring the photocathode to the NEA surface state, continued emission of electrons may in some cases lead to electrons being emitted while the energy level of the vacuum level of the photocathode returns to a high level from a level lower than the energy level of the conduction band. Consequently, in cases in which the photocathode is used as an electron-beam source, it is preferable to lower the energy level of the vacuum level of the photocathode to the greatest extent possible; however, it is not essential to bring about or maintain the NEA surface state. Accordingly, in the present invention, “treatment to lower electron affinity” refers to a treatment to lower the energy level of the vacuum level of the photocathode to a level at which electrons can be emitted. Herein, “treatment to lower electron affinity” is sometimes referred to as “EA surface treatment,” and a state in which, through “treatment to lower electron affinity,” the energy level of the vacuum level of the photocathode has been lowered to a level at which electrons can be emitted, as an “EA surface.”
An EA surface is degraded by adsorption of trace amounts of H2O, CO, CO2, or other residual gases, or by backflow of ionized residual gases onto the EA surface. For this reason, in order to stably output an electron beam from a photocathode for an extended period, an ultrahigh degree of vacuum is necessary for treatment and maintenance. Moreover, the quantity of electrons output by an EA surface-treated photocathode is finite, and once a given quantity of electrons has been emitted, it is necessary to again subject the photocathode surface to EA surface treatment.
FIG. 3 is an entire photo of an electron gun 10 that uses a conventional EA surface-treated photocathode. The electron gun 10 includes at least an EA surface treatment chamber 11, an electron gun chamber 12, and transport means 13 for the EA surface-treated photocathode. As described above, it is necessary for the EA surface-treated photocathode to be loaded into the electron gun without being exposed to the outside air while continuing to maintain an ultrahigh vacuum state after carrying out EA treatment in an ultrahigh vacuum. Additionally, once a given time period has passed, it is necessary to repeat the EA surface treatment of the photocathode, and in the prior art, it was necessary to furnish an EA surface treatment chamber and an electron gun chamber separately. The reason was that in conventional EA surface treatment there is adopted a method involving vapor deposition of a surface treatment material directly onto the photocathode inside a chamber, but when EA surface treatment is carried out in the same chamber, the EA surface treatment material is deposited onto the electron gun chamber and various devices inside the chamber, and EA surface treatment material deposited near the electrodes in particular can lead to the occurrence of field emission dark current, dramatically lowering the capability of the electron gun.
However, in cases in which the EA surface treatment chamber and the electron gun chamber are furnished separately, firstly, two chambers brought to an ultrahigh vacuum state are needed; and further, the transport means 13 is needed to transport the photocathode treated in the EA surface treatment chamber into the electron gun chamber while continuing to maintain the ultrahigh vacuum state, leading to the problem that the electron gun device becomes extremely large. Moreover, because it is necessary for the EA surface-treated photocathode to be moved from the EA surface treatment chamber to the electron gun chamber and installed therein while maintaining an ultrahigh vacuum, and also to be moved from the EA surface treatment chamber to the electron gun chamber and installed therein during EA surface re-treatment of the photocathode, the device must be designed precisely, and proper manipulation of the photocathode is necessary to avoid dropping it during transport, leading to the problem of complicated device management.    [Non-Patent Document 1] MRS-J NEWS, Vol. 20, No. 2, May 2008